Intermittent Single-Molecule Interfacial Electron Transfer Dynamics Vasudevanpillai Biju, Miodrag Micic, ² Dehong Hu, and H. Peter Lu* Contribution from the Pacific Northwest National Laboratory, Fundamental Science DiVision, P. O. Box 999, Richland, Washington 99352 Received February 17, 2004; Revised Manuscript Received May 10, 2004; E-mail: peter.lu@pnl.gov Abstract: We report on single-molecule studies of photosensitized interfacial electron transfer (ET) processes in Coumarin 343 (C343)-TiO2 nanoparticles (NP) and Cresyl Violet (CV + )-TiO2 NP systems, using time-correlated single-photon counting coupled with scanning confocal fluorescence microscopy. Fluorescence intensity trajectories of individual dye molecules adsorbed on a semiconductor NP surface showed fluorescence fluctuations and blinking, with time constants distributed from milliseconds to seconds. The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial ET reaction dynamics. We attribute fluorescence fluctuations to the interfacial ET reaction rate fluctuations, associating redox reactivity intermittency with the fluctuations of molecule-TiO 2 electronic and Franck-Condon coupling. Intermittent interfacial ET dynamics of individual molecules could be characteristic of a surface chemical reaction strongly involved with and regulated by molecule-surface interactions. The intermittent interfacial reaction dynamics that likely occur among single molecules in other interfacial and surface chemical processes can typically be observed by single-molecule studies but not by conventional ensemble-averaged experiments. Introduction Interfacial electron transfer (ET) processes play an important role in many chemical and biological processes. 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